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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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HIDDEN, FOURTH AND SMALLER GLACIERS                  151
Farther out only pits were formed at first, being filled later by alluviation; and this process might go on for years. The ultimate kettles upon the pitted plain would be those due to the melting of the last isolated remnants of the buried glacier that were not filled by the last alluviation.
The outwash gravel plain built in front of the Hidden Glacier furnishes an excellent example of a deposit common in regions of Pleistocene glaciation in Europe and America, but here in actual process of formation. The significance of the phenomena observed in this valley in 1905 will be apparent to students of Pleistocene glacial deposits. If the ice had all melted away in 1905 there would have developed a crescentic kame moraine area with an ice-contact face foward the glacier and with a sloping outwash gravel plain toward the sea, pitted by numerous kettles. The stream channels on the plain would extend up to where the glacier front stood in 1905. Many of the kettles would be filled.
Recession Between 1899 and 1905. The photographs and map made by the Harriman Expedition in 1899 give a basis for fairly exact comparisons of the conditions in 1899 and 1905. In some cases it was possible to occupy the exact sites of Gilbert's 1899 pictures (PI. LXXVII); in other cases the approximate position could be located. In the interval of six years there was a recession of the Hidden Glacier front of more than a quarter of a mile. There was evidently at the same time a change in position of the most projecting part of the visible ice front, for Gannett's map shows the outermost point close to the south wall of the valley, whereas in 1905 the glacier projected most near its center. There was also a change in the point of emergence of the largest glacjal stream, as already stated. The lateral moraine on the south side was more pronounced in 1905 than in 1899, but this fact is probably only partly due to continued recession, for our visit was about a month later in the season than Gilbert's, and naturally by that time summer ablation would have brought the moraine into greater relief.
Earlier Recession. Prior to 1899 we have no sufficiently accurate basis for determining the position of the Hidden Glacier front, though the Canadian Boundary Commission map, based upon observations in 1895, and Russell's name Hidden Glacier, given in 1891, both indicate that the glacier front was in those years well back within the mountain valley. The glacier was less than If miles from the fiord on the 1895 map, and, if this is accurate, it retreated about 600 feet between 1895 and 1899. However, that Hidden Glacier had recently been much more extensive than in 1899 was clearly proved by a number of facts, notably the pronounced grooving and fluting of the mountain slopes, to an elevation of 1000 or 1500 feet, of such recent date that weathering had not notably erased it; the general absence of vegetation on the lower valley slopes and the entire absence of alder thickets; and the presence of overridden gravels.
The north side of Hidden Glacier valley, which has not been as greatly oversteepened by glacial erosion as the south side, had in 1899 and 1905 a series of gravel deposits extending two or three hundred feet above the outwash plain and down the valley for a mile and a half. The gravel surface was sculptured in indistinct, westward-descending terraces, and stream cuts showed excellent stratification with fragments of trees among the gravels.
The north end of the glacier rested upon these gravels (PI. LXXX, B), the glacier surface sloping 10 to 15 westward and the gravel surface about 5 southeastward, or toward the centre of the glacier, causing the ice to wedge out into a thin edge. It was